This invention relates to lubricants, and, more particularly, to a dry film lubricant for use between contacting titanium pieces.
When materials rub or slide against each other during service, the resulting frictional forces can cause damage to the materials through the generation of heat or through a variety of surface deformation or fatigue processes generally termed fretting. These adverse effects can be reduced by reducing the coefficient of friction between the surfaces, by the application of a lubricant. The reduction of friction results in lower heat production, reduced stress on the contact faces, reduced surface deformation, lower fatigue forces, and, consequently, lower fretting damage. Many types of solid, liquid, and gaseous lubricants are known and widely used.
One of the most challenging lubrication problems is encountered with titanium surfaces that rub against each other. In one such situation, a titanium compressor or fan rotor in an aircraft engine has an array of dovetail slots in its outer periphery. The dovetailed base of a titanium compressor or fan blade fits into each dovetail slot. When the disk is at rest, the dovetail of the blade rests within the slot. When the disk is rotating during engine operation, the blade is drawn radially outwardly by centrifugal forces so that the sides of the dovetail of the blade ride against the inwardly sloping sides of the dovetailed slot of the rotor, producing localized elevated stress conditions on the contact surfaces.
There is movement between the two titanium parts with changes in engine operating conditions such as speed and temperature, and as the engine passes through cycles of startup and shutdown. With repeated cycles of operation, this movement becomes a fatigue process. During such low-cycle fatigue, the forces exerted on the rotor in the neighborhood of the dovetail slot can lead to the introduction and propagation of fatigue cracks in the rotor, and eventually to failure, resulting in shortened rotor life. Such fretting damage is of particular concern with titanium parts.
The incidence of fatigue damage can be reduced by lubricating the contact region between the titanium pieces. Liquid lubricants are not practical, because of the operating temperatures of 200.degree.-600.degree. F., nor are gaseous lubricants practical because of the high frictional loads. It has been common practice to deposit low-friction coatings on the titanium pieces, and to use dry film lubricants such as mixtures of molybdenum disulfide and an oxide of antimony between the pieces. However, these lubricants degrade relatively quickly in turbine engine applications due to the high loading stresses, leaving the wear surfaces inadequately lubricated. The lubricants may be reapplied during maintenance.
While these measures are effective in reducing frictional loadings and frictionally induced damage, at least until lubricant degradation occurs, there is a continuing need for improved lubricants. Such improved lubricants would be operable to reduce fretting damage between titanium pieces for larger numbers of cycles, resulting in reduced maintenance requirements, and produce more consistent operating performance. The present invention fulfills this need, and further provides related advantages.